Assuming you're the villian, or just have 0 resistance and Garen has maxed R.
If 1100HP is 60% of your full HP, then your full HP was at 1100/(0.6) = 1833.33.
For Garen's ulti to do 1100 damage, so 525 + x*0.4 = 1100, where x refers to the amount of MISSING health. So x = 1437.5.
In other words, you must have only 1833.33 - 1437.5 = 395.83 HP LEFT, assuming you're also the villain so it does true damage (or having 0 resistance so it basically does true damage), for the ulti to deal 1100 damage to you; which means you were already 1 step from death anyway, especially in later stages of the game, so not like it really matters.
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If you were not the villian, or that you have some resistances, and here let's assume you have 100 MR, since you said you were a fighter.
It means the damage multiplier the damage receives upon contact is 100/(100+100) = 0.5. So Garen's ulti damage would be halved.
So in other words, Garen's ulti must've dealt 1100*2 = 2200 for it to have 1100 damage on you.
525+x*0.4 = 2200 => where x is solved to be 4187.5, which is impossible because that means you need to lose more than twice of your own max HP (1833.33 as calculated before) for them to do that damage.
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I wouldn't think there would be that much difference to calculate for the cases of un-maxed R because it will probably just return more cases where you have to lose more HP than your max before Garen can deliver the amount of damage on your HP bar then proposed in this test case.
So yeah, based on these calculations, I wouldn't say at least in this case, Garen was being too oppressive.